Microfabrication, also known as micromachining, commonly refers to the use of known semiconductor processing techniques to fabricate devices known as micro-electromechanical systems (MEMS) or micromachined devices, such as micromachined inertial sensors. In general, known MEMS fabrication processes involve the sequential addition and removal of layers of material from a substrate layer through the use of film deposition and etching techniques until the desired structure has been realized.
Single-crystal silicon (SCS) is an often-desired substrate material for MEMS devices because of its excellent mechanical properties and IC-compatibility. For inertial sensors, bulk silicon also provides thick flat structures and large mass, which can lead to high sensitivity and high resolution. In addition, bulk silicon provides the feasibility to make relatively large, flat scanning micromirrors for medical imaging applications. In MEMS devices, bulk silicon may serve as (i) electrodes, (ii) active sensing or actuation elements, as well as (iii) support structures. Therefore, electrical isolation of bulk silicon is required.
It is known in the art to use complementary metal-oxide-semiconductor (CMOS)-compatible fabrication processes to create microstructures (or MEMS structures). Such processes are disclosed in U.S. Pat. No. 5,717,631 to Carley et al., U.S. Pat. No. 5,970,315 to Carley et al., and U.S. Pat. No. 6,458,615 to Fedder et al., each of which is incorporated herein by reference. CMOS-MEMS processing creates microstructures, such as beams, that are made out of the dielectric and metallization layers of CMOS and/or substrate material. One of the CMOS metal layers (or some other layer made from an etch-resistant material) acts as an etch-resistant mask for defining the microstructural sidewalls. A reactive-ion etch of the CMOS oxide layer then creates composite metal/dielectric/substrate material microstructures that can have a high aspect ratio of beam width to beam thickness, and of gaps between adjacent beams to beam thickness. To electrically isolate the substrate material, an isotropic etch of the substrate may then be used to remove the substrate material from under a special composite microstructure (e.g., a short, narrow beam). As a result, the substrate material on both sides of the microstructure is electrically isolated but mechanically connected.
The isotropic etching step, however, has the effect of also removing substrate material from under other structures of the device. The extent of this undercutting depends on the etch time and on the width of the beam being released: the wider the beam the larger the required undercut. This undercutting may not be desirable for other structures of the device, such as interleaved comb fingers, for example, where it is desirable to have as small as undercut as possible. Thus, in many device designs there is conflict between the requirements to undercut in different areas.